Atom Interferometry for detection of Gravity Waves-a

This report presents the results of the 2012-2013 NASA Institute for Advanced Concepts (NIAC) Phase 1 Atom Interferometry for Detection of Gravitational Waves project. The origin of this GW detection concept using atoms can be traced to theoretical work that first appeared in 2008 [2] and also to a satellite mission-focused follow-up study that was done in 2011 [3]. The goal of the current project was to explore both theoretical and technical issues surrounding the implementation of this idea, as well as to begin performing proof-of-concept experiments to validate critical aspects of the proposal. The top level trade space for the detector design is driven by the strategy employed to mitigate laser frequency noise, which, if uncontrolled, can mask GW signatures. One of the advantages of the atom interferometric approach is the possibility of single baseline detection (Fig. 1.1), even in the presence of laser noise. This is enabled by the differential measurement between the two ensembles of atoms, which can result in substantial laser noise suppression. The details of this suppression depend on the atomic physics techniques used to implement the atom interferometry. Specifically, we considered the effect on noise suppression that results from using traditional two-photon Raman transitions (with alkali atoms) and also single-photon transitions (with alkaline earth-like atoms). The interferometers shown in Fig 1.1(b) take advantage of single-photon transitions (as opposed to traditional Raman transitions) because using light pulses from one dir ection at a time allows for near perfect common-mode cancellation of laser phase noise, even for long baselines.[1] This calls for the use of atomic transitions with an (ideally large) optical energy level difference with a long (> 1 s) lifetime, such as high- trans itions routinely used for optical atomic clocks in species like Sr, Ca and Yb. Notably, large momentum transfer (LMT) atom optics[8] - and the sensitivity enhancement they confer - can still be realized by simply adding additional pairs of alternating pulses to each beamsplitter process.[1] Section 3 reports on the theoretical work we performed to justify this GW detection protocol using single-photon transitions. This approach repr esents a new method for GW detection using atoms that is distinct from the original proposal from 2008. At the system level, we evaluated three architectures, each of which implements a different solution to the laser frequency noise issue. The first two designs are based on two-photon Raman transitions with Rb atoms. One of these is a three-satellite, multiple baseline design while the other is a two-satel-lite, single baseline design. The third proposal is a two-satellite, single baseline design that uses single-photon transitions with Sr atoms. These three architectures are described in more detail in Section 2. There are a number of known technical issues that we have started to address using ground-based experiments. These issues include atom technology development needs such as, for example, lower ensemble temperature requirements and large momentum transfer (LMT) atom optics. To this end, we have built a 10-meter scale atom drop tower[9, 10] where we can perform proof-of-principle dem-onstrations of the proposed AGIS detector in an environment that permits more than 2.5 seconds of free-fall time. This facility allows for demonstration of atom interferometry with long interrogation time (seconds) and large atom wavepacket separation (meters), which is the regime required for GW detection at scientifically interesting levels. Sections 4 and 5 describe the results of these experiments. Section 4 describes the demonstration of an atom interferometer at high contrast with a record interrogation time of 2T =2.3 seconds, as well as a new technique for evaluating and controlling ve locity dependent systematic phase shifts that typi-cally cause inhomogeneous broadening that reduces interferometer contrast. Section 5 is a discussion of a new interferometer phase readout procedure that we developed. This new technique, called Phase Shear Readout (PSR) allows the phase and contrast of the interferometer to be measured with a single shot. PSR has the potential to offer a dramatic reduction in a variety of noise requirements for the GW detector, including satellite rotation stability and optical wavefront aberrations.